1. Field of the Invention
The present invention relates to optical signal monitoring techniques and in particular to a signal quality monitoring method and apparatus in an optical wavelength division multiplexing (WDM) network.
2. Description of the Related Art
One conventional method of monitoring the quality of a transmission signal is to monitor the code error ratio thereof by executing the parity error check of standardized overhead information which is provided in a transport frame in a new synchronous network such as SDH (Synchronous Digital Hierarchy).
Due to the recent vast increase in information transmission capacity, people desire an optical network using the wavelength division multiplexing (WDM) technology. This optical network using the WDM technology can provide communication services that do not depend on the signal transmission speed and format, and is receiving a great attention for its transparency to the signal transmission speed and format.
To manage the quality of transmission signals in such an optical network, it is necessary to monitor the quality of transmission signals using a method suitable for the optical network, as in the case of the new synchronous network. One of such methods has been proposed in, for example. International Conference on Communication held in 1997, entitled xe2x80x9cPhotonic Transport Network Architecture employing Optical Path Concept,xe2x80x9d Kimio Oguchi and et al., ICC"" 97 workshop, Jun. 8-12, 1997. It is to monitor the code error ratio by performing the parity error check of standardized overhead information which is provided in a transport frame, as in the case of the new synchronous network.
The aforementioned prior art however suffers the following problems. The first problem is the necessity to always affix standardized overhead information into a transmission signal. This restricts the format of a transmission signal, disabling to make the best use of the transparency of an optical network using the WDM technology. The second problem lies in the indirect monitoring method because a transmission light signal is not directly monitored itself, but rather monitoring is implemented by checking bit parity in the overhead information.
There has been proposed a digital data receiver which can detect fault occurrence of a transmission line or a transmitter without the need of a complicated parity detector (see Japanese Patent Unexamined Publication No. 61-29243).
More specifically, the digital data receiver is provided with first and second decision circuits and an exclusive-OR circuit connected to the output terminals of the first and second decision circuits. The first decision circuit is provided with a first comparator and a first flip-flop circuit clocked by a clock signal extracted from the input signal. The first comparator compares an input signal with a first reference voltage which is set so as to minimize the bit error rate thereof. The output of the first comparator is output to the first flip-flop circuit. The second decision circuit is provided with a second comparator and a second flip-flop circuit clocked by the clock signal extracted from the input signal. The second comparator compares the input signal with a second reference voltage differences from the first reference voltage. The output of the second comparator is output to the second flip-flop circuit.
When the voltage level of the input signal drops below the first reference voltage, the outputs of the first and second flip-flop circuits do not coincide and such a mismatch is detected by the exclusive OR-circuit.
The conventional digital data receiver, however, use the first and second comparators to monitor a change in amplitude of the input signal.
It is an object of the present invention to provide a signal monitoring method and apparatus, which can provide more reliable light transmitting/receiving capability to an optical network.
According to an aspect of the present invention, a method for monitoring quality of a light signal having a data transmission rate in an optical network, comprising the steps of: a) extracting a clock signal from the light signal; b) setting a decision region for each bit of the light signal, wherein the decision region is defined by a plurality of threshold amplitudes corresponding respectively to threshold phases which are produced with respect to the clock signal; and c) monitoring the quality of the light signal depending on whether an amplitude of the light signal falls into the decision region at timings of the N threshold phases.
The step b) comprises the steps of: producing N (N in an integer greater than 1) threshold phases with respect to the clock signal, wherein the N threshold phases provide different timings for each bit of the light signal: and producing N threshold amplitudes which correspond to the N threshold phases, respectively.
The step c) preferably comprises the steps of: comparing an amplitude of the light signal with the N threshold amplitudes at timings of the N threshold phases to produce comparison results, respectively: counting error bits for a predetermined time period based on the comparison results; and monitoring the quality of the light signal by comparing a count of the error bits with a predetermined value.
When the count of the error bits is greater than the predetermine value, a signal quality degradation alarm may be generated.
According to another aspect of the present invention, an apparatus includes a light receiver for receiving the light signal to produce an input signal which is an electric signal corresponding to the light signal; N bit decision circuits for making a bit decision with respect to different decision thresholds for each bit of the input signal to produce N bit decision results, respectively; and a monitor for monitoring the quality of the light signal based on the N bit decision results. Each of the N bit decision circuits comprises: a clock recovery circuit for extracting a clock signal from the input signal; a phase shifter for phase-shifting the clock signal by a predetermined amount to produce a phase-shifted clock signal; and a bit discriminator for making a binary discrimination of an amplitude of the input signal with respect to a predetermined threshold voltage at timing of the phase-shifted clock signal to produce a bit decision result, wherein a decision threshold is defined by the phase-shifted clock signal and the predetermined threshold voltage.
The bit discriminator may be a D-type flip-flop circuit having differential input terminals and at least one output terminals, wherein the input signal is applied to a first input terminal and the predetermined threshold voltage is applied to the second input terminal.
The monitor may count error bits for a predetermined time period based on the N decision results and determine the quality of the light signal by comparing a count of the error bits with a predetermined value.
According to further another aspect of the present invention, a fault recovery method in an optical network composed of a plurality of nodes each providing a plurality of paths, wherein a light signal having a data transmission rate is transmitted through a path of a node, comprising the steps of: at each of the nodes, a) extending a clock signal from the light signal; b) setting a decision region for each bit of the light signal, wherein the decision region is defined by a plurality of threshold amplitudes corresponding respectively to threshold phases which are produced with respect to the clock signal; c) monitoring the quality of the light signal depending on whether an amplitude of the light signal falls into the decision region at timings of the N threshold phases; and d) switching the path from a working path to a protection path when the quality of the light signal drops below a predetermined quality level.
According to still another aspect of the present invention, an extinction ratio of the light signal may be obtained. An apparatus includes a light receiver for receiving the light signal to produce an input signal which is an electric signal corresponding to the light signal and first and second bit decision circuits for making a bit decision with respect to different decision thresholds for each bit of the input signal to produce first and second bit decision results. Each of the bit decision circuits comprises: a clock recovery circuit for extracting a clock signal from the input signal; a phase shifter for phase-shifting the clock signal by a predetermined amount to produce a phase-shifted clock signal; and a bit discriminator for making a binary discrimination of an amplitude of the input signal with respect to a threshold voltage at timing of the phase-shifted clock signal to produce a bit decision result, wherein a decision threshold is defined by the phase-shifted clock signal and the threshold voltage. The apparatus further includes a logic circuit for calculating an exclusive OR or the bit decision results to produce an error bit; an error counter for counting error bits for a predetermined time period; a voltage controller for controlling the threshold voltage of each of the bit decision circuits so as to minimize a count of the error bits; and a circuit for calculating an extinction ratio obtained by dividing a first threshold voltage of the first bit decision circuit by a second threshold voltage of the second bit decision circuit.